In printing pictorial matter, whether by lithography, letterpress or gravure, the half-tone process is used, wherein the actual printing image is composed of thousands of minute dots per square inch of a single color ink of varied dot size or ink density. What the naked eye sees as shading in half-tone prints is actually controlled variation in the size of dots relative to the unprinted areas between dots. In black and white pictorial matter the dots are printed in black ink only. Full color reproductions, however, are necessarily printed in each of at least three colors, cyan, magenta, and yellow (known as "three color process"), or in these same colors with the addition of black ("four color process"). For each color a separate printing plate is made. In order to make the three or four printing plates, the original color picture or photograph is "separated" photographically, with the use of filters, masks, etc., into a set of three or four half-tone negatives, each representing one of the colors, and containing, dot for dot, the amount of that color which must be printed to produce the desired total color print. The preparation of the color-separation negative is an art and requires considerable skill in handling of the many variables to produce a desired result. Often trial and error is involved requiring correction or replacement of one or more of the negatives. Unless some reliable system is available to "proof" the negatives, the printing press must be set up and copy printed just to secure preliminary proofs. This is time consuming and expensive. It is therefore desirable to provide accurate means for proofing the negatives without printing.
One system for proofing color separation negatives is disclosed in U.S. Pat. No. 3,136,637. A light-sensitive transparent sheet is provided for each of the colors to be printed. Each of the sheets is exposed through its respective color separation negative. Upon processing, the color in the non-image areas is removed, yielding a sheet which contains the desired color pattern in the image areas, while being colorless and transparent in the non-image areas (e.g. between the halftone dots). After each of the separate sheets is made, they are assembled together in registry on a white background, whereupon a color proof results.
That system of color proofing has a number of inherent drawbacks. For example, the laying up of the multiplicity of sheets requires that the viewer look through a plurality (three or four) of transparent films during the proofing operation. Since the composite is made of several separate sheets, extreme care is required to maintain registry. If the individual sheets are not perfectly colorless and transparent in the optical sense, any "haze" or imperfection present is multiplied in the several sheets. Additionally, incident light reflects from the several sheets imparting a gloss which is not truly representative of printed copy, thus introducing a need for interpretation in evaluating the proof.
U.S. Pat. No. 3,671,236 improves upon the proofing system described in U.S. Pat. No. 3,136,637. Photomechanically produced images corresponding with each color are integrally built up on a single substrate (much as occurs in the actual printing operation itself) without any printing operations. The multiplicity of carrier films is eliminated by transferring individual color images from a sheet comprised of (1) a carrier with release surface, (2) pigment and binder layer, (3) photohardenable or insolubilizable layer, (4) barrier layer and (5) pressure-sensitive adhesive layer.
Further improvement over U.S. Pat. No. 3,371,236 is made in U.S. Pat. No. 4,656,114 by the replacement of the pressure-sensitive adhesive layer by a thermally laminable adhesive layer. A thermally adhesive layer can be positioned (before lamination) without a chance of altering the receptor surface or picking up extraneous matter which can occur with pressure-sensitive layers. The physical nature of the smooth, transparent and optically clear thermal adhesive layer offers better optical qualities than the less physically smooth pressure-sensitive adhesive layer.
Blocking is the undesirable adhesion between touching layers such as that which occurs under moderate pressure or during storage. There is often impairment of the adhesive or the contacted surface when an attempt is made to separate members that have bonded. It has been conventional practice to add antiblocking agents to adhesive layers, and in some instances to the opposed surfaces. Typical antiblocking agents include particulates (especially silica), and soluble organic materials that dissolve in the adhesive. silicas can cause significant haze and the soluble antiblocking agents often do not coat out well in certain adhesives and can solubilize important ingredients in adjacent layers. This last problem is particularly damaging in imageable layers with critical sensitometric and color balance problems such as prepress color proofing elements.
The use of silica particles as antiblocking agents is well known. The use of silica particles is well known. Cab-o-sil.TM. fumed silica is suggested by Cabot Corp. in their product use literature for inclusion in all types of adhesives (water, solvent and heat). When the coatings are activated good adhesive properties result. Nominal particle diameters are less than 0.1 micron (0.01 to 0.07). W. R. Grace, in their description of Syloid.TM. silicas suggest silicas for elimination of blocking and indicate that controlled particulate size distribution lowers concentrations required for antiblocking. Particle sizes average from 2.5 to 15 microns.
U.S. Pat. No. 2,77,247 shows that preferred polyethylene antiblocking agents have molecular weights not lower than 4000 or greater than 10,000, are used in quantities varying between 2 and 10% by weight (preferred 4 to 8) dissolved in adhesive layer.
Dusting with plasticizer to produce a non-tacky film in U.S. Pat. No. 2,678,284 provides a method of quickly and easily producing a nontacky film of thermoplastic adhesive. The chilled adhesive coated film is dusted with Santizer 3 in the form of a finely divided powder with particles less than about 10 microns.
"Hot-Melt Adhesives" by M. McDonald, Noyes Data Corp. 1971 states that "considerable difficulty has been experienced with coated hot-melt adhesives because of the tendency to adhere or "block" during warm, humid weather. (discusses U.S. Pat. No. 2,772,247 above).
U.S. Pat. No. 3,343,978 describes a non-tacky layer used as a heat-activated adhesive. A typical adhesive (e.g., Example 3) comprises polyethylene, polyterpene resin, and a wax antiblocking agent. This shows the use of a continuous phase (dissolved) antiblocking agent.
U.S. Pat. No. 4,719,169 shows the application of an anti-blocking layer on a proofing system. It is not a particulate layer and may be coated over an adhesive layer, on the back side of the system, or under the adhesive layer (in a transfer system).
U.S. Pat. No. 4,221,862 describes a method for producing finely divided poly(methylmethacrylate) (PMMA) particles. These PMMA beads may be in an aqueous dispersion and have a fairly narrow size distribution.
U.S. Pat. No. 4,522,967 describes a heat-sealable water dispersible adhesive comprising an antiblocking agent at a 5 to 35% by weight level in the adhesive. The anti-blocking agents are selected from polyalkylene glycol materials.
U.S. Pat. No. 3,531,316 uses a powdered insoluble polymer in a thermoplastic layer overcoating a solvent activated adhesive layer. The powdered polymer prevents blocking by said adhesive layer. The antiblocking layer comprises a thermoplastic film forming resin and 50 to 80% by weight of the insoluble polymer powder. A solvent is used to activate the undercoated adhesive layer.